Suppressor devices have become a cornerstone of Suppressed Conductometric Ion Chromatography (SCIC) [1] systems, which are used for the separation and determination of inorganic and many small organic ions [2]. Since electrical conductivity is a universal property of ions, conductance measurement is the most commonly used detection technique in ion chromatography (IC) systems, where an ionic eluent at millimolar concentrations is used to elute analyte ions, typically present at micromolar (μM) concentrations (˜3 orders of magnitude less than the eluent concentration), from a separation column. Because the equivalent conductance of the eluent (λE−) and the analyte ions (λA−) differ, it is possible to conductometrically detect eluting ions directly after the separation column (Non-Suppressed Ion Chromatography (NSIC)), but the sensitivity is poor as the difference in conductivity |(λA-λE−)| is small and the background conductivity is high. A suppressor serves to convert the eluent to a very weak electrolyte (weak acid/base or water), greatly reducing (suppressing) the background conductivity caused by the eluent. Analyte signals are also enhanced because their counterions are substituted by more conductive hydronium or hydroxide ions. By reducing the background conductivity (and hence noise level) by ˜2 orders of magnitude and improving the analyte sensitivity by nearly an order of magnitude, the limits of detection (LODs) of SCIC systems can be improved by 2 to 3 orders of magnitude relative to NSIC.
A variety of suppressors have been developed for such systems, from early packed-column [1], hollow-fiber [3,4], and micromembrane suppressors [5], to modern electrodialytic membrane-based [6,7] and continuously regenerated packed-column suppressors [8], as well as colloidal ion exchangers [9]. Microfluidic suppressors have also been described [10-12]. By exploiting the electrolytic decomposition of water to generate the hydronium or hydroxide ions necessary for suppression reactions, an electrodialytic membrane-based suppressor can be operated in self-regenerating mode without the addition of any regenerant, and permits high dynamic suppression capacity with a low dead volume.
Capillary ion chromatography (CIC) is gaining attention because of its low sample and eluent consumption and high efficiency. Suppressed Conductometric Capillary Ion Chromatography (SCCIC) was first demonstrated by Rokushika et al. [13], who coupled a resin-packed fused-silica capillary column (0.19 mm i.d.) to a sulfonated hollow fiber tube (0.2 mm i.d.×10 mm) functioning as a suppressor. CIC technology was last reviewed by Kuban and Dasgupta [14]. Strategies used in larger-scale systems, such as integration of the suppressor and detector [15], can reduce post-suppressor broadening but do little for broadening in the suppressor and do not address the issue of dispersion in any connection between the separation column and the suppressor. Strategies previously used to reduce broadening in macroscale suppressors, such as packing perfluorosulfonate cation exchanger (Nafion®) tubing with inert beads [16], filling with nylon monofilament [4], etc., are simply inapplicable in CIC-scale systems.